Unzen as a day trip from Kumamoto?

Hi All,
I will be travelling in Kryushu in December and I am just working out my itinerary. Generally we prefer to stay in fewer places and do day trips from there rather than pack and travel each night. We were going to visit Kumamoto and do a day trip from there to Aso, but I am concerned it might be inaccessible (as it is currently and I think it has been for a while inaccessable). So we were thinking of visiting Unzen by ferry as an alternative? Is this an easy day trip to do or would it be rushed? Is it easy to get a bus to Unzen from Shimabara pier without too much waiting around? Or should we plan to stay at least 1 night there? We don't plan to visit Nagasaki this time around. Our other base cities will be Kagoshima and Fukuoka. We don't think we will get to Beppu area, so we're interested to go to Unzen mainly to see the bubbling 'hells', the volcano, steam coming out of the ground etc.
Thanks for your help.
Alexis.

It would be doable, but also maybe a pain to do. Taxi from Kumamoto to port. Ferry from Port to Shimabara. Bus to Unzen.

http://www.shimatetsu.co.jp/

These is some bus and train timetables on this website. From memory that buses are hourly from the Shimabara port area.

Personally I wouldn't do it if coming from Kumamoto. I'd probably catch the shinkansen to Fukuoka or Kagoshima and do something I've missed. O train ride the Hisatsu Line to Hitoyoshi or Orange Line and sight see, then return via Sendai.

Taking the ferry across to Shimabara would give a different perspective on getting around. I've done the trip you propose several times in 4-5 hours but have had my own car transported over. To save the RT ferry car costs, you could rent a car at Toyota in Shimabara. This would give you the most freedom to go and do what you want such as stop at Nita-toge for the cable car up, the volcano museum where the pyroclastic flows came through, as well as the hot springs area.
The Toyota rental is a few blocks north of the ferry:
https://rent.toyota.co.jp/shop/detail.aspx?rCode=68301&eCode=008&udFlg...

You could also hire a "kanko taxi" (sight-seeing taxi) after you got off the ferry. For several people, this is an economical option and although a bit structured, it isn't the same as a non-stop bus and you know you won't be sitting at a bus stop for 45 minutes wondering where the bus is.
http://www.shimatetsu.co.jp/kiji/pub/detail.aspx?c_id=40&id=111&pg=1

Kumamoto-Shimabara Ferry schedules (2 companies)

Ocean Arrow Ferry:
http://www.kumamotoferry.co.jp/lang_en/
Kyusho Ferry:
http://www.fune.co.jp/J-index/Z-teiki/JZ-o/JZ-O-59.html

I would also recommend driving along the coast a bit, especially on the SW side as there are scenic villages and hilly peninsula jutting into the sea. Also on the SE side are the ruins of Hara Castle, site of the famous peasant revolt in 1637.
Shimabara:
http://www.japanvisitor.com/japan-city-guides/shimabara

There is an irregular Sanko bus from Kumamoto Station to the port sheduled to match the ferries.
Weekday schedule:
http://www.kyusanko.co.jp/sankobus/ttsys/timetable.php?Yyyymmdd=201605...

Hi, the sight seeing taxi option sounds good but the website is in Japanese, sorry I don't speak or read Japanese at all. Is it possible to rent a taxi for the day there as a non Japanese speaker? I am worried about renting a car as I won't be able to read the road signs and I won't have a clue where I am going. That's why I was hoping to just arrive by ferry and then catch a bus straight to Unzen onsen and have a walk around then come back the same way. I would like to go to the museum as well. Perhaps I need to book a night there to give more time?
Alexis.

Unable to read road signs? Your English looks pretty good to me.

Thanks for your less than helpful comment Hakata. Obviously I was referring to signs that may be in Japanese. I have also received mixed reports on whether or not GPS with voice in English is available at hire car companies in Kryushu, do you know if this is available at Toyota at Shimabara?

Reserve a car with a multi-lingual "Navi."
https://rent.toyota.co.jp/eng/service/navi.html

However, for the Shimabara Peninsula, you'd have to be pretty geographically impaired to not be able to find your way around. Most signs now are in both Japanese and Romaji (Roman letters-some people say "English" but it's not). I usually use a map because a "navi" doesn't give you the big picture and you can miss things along the way being guided blindly (or I just zoom way out on the navi so I can see the entire route + options). They are best used in cities, but there's not much urban there except Shimabara City but if you got lost there you'd be out of town before you got turned around. Most of the entire Shimabara Pen. is rural with small towns. The main roads have the coast on one side and the mountains on the other. When I rent a car that has one, I just make sure I can get the location of the rental company to show up on it before I leave the rental place.

Thanks that's helpful to know. Is it possible to get a map from the hire company or the port? I think I have been put off driving in Japan before but it sounds like it would be a good option here. Thanks.